The heart actively coordinates cardiovascular responses through release of hormones. Ventricular proenkephalin mRNA content is twice that of brain suggesting that enkephalins function as cardiac hormones. We find enkephalins evenly distributed throughout dog heart, but proenkephalin is 3-5 X higher in ventricles than in atria and proenkephalin content is 10 X higher than enkephalin. This differs from norepinephrine which is concentrated in the atria and suggests that enkephalin is not stored in cardiac nerves. We also describe immunofluorescence in ventricular tissue stained with a proenkephalin antibody. The fluorescent pattern depicts proenkephalin within myocytes and was clearly distinct from patterns associated with ventricular innervation. Myocardial enkephalins declined sharply following hemorrhage and increased after denervation and ganglionic blockade. In addition, repeated stimulation of the post ganglionic cardiac nerve leads to an opioid dependent decline in coronary overflow of norepinephrine. Together, these observations indicate that neuronal transmission initiates an opioid feedback loop within the heart with the release of opioids during autonomic activation and their accumulation following ganglionic blockade. We believe cardiac opioids are important governors of myocardial excitability. Excess opioids might contribute to cardiac depression as observed in shock; conversely, insufficient opioids may lead to increasing irritability causing arrhythmias. Our results demonstrate myocardial enkephalins respond to alterations in autonomic function which leads to our hypothesis that autonomic input to the heart regulates synthesis and secretion of cardiac enkephalins. We propose to collect hearts from animals with control, diminished and augmented autonomic activity to: A) Characterize Enkephalin Secretion: Cardiac tissue will be perfused to evaluate the content and secretion of enkephalin and proenkephalin in response to electrical, ionic and hormonal stimuli; B) Extract and Identify Cardiac Enkephalins and Proenkephalin mRNA: Enkephalins will be extracted from cardiac tissue and chromatographically identified with attention to relative content of precursors vs products. Proenkephalin mRNA will be quantitated by Northern blot analysis. C) Localize Enkephalins Anatomically: Cardiac tissue will be prepared for immunohistochemistry utilizing immunogold techniques and antibodies recognizing proenkephalin and enkephalin. Together, these studies should provide important information on the role of opioids and their autonomic regulation in heart function.